This invention relates generally to radio frequency circuits and more particularly to frequency conversion circuits.
As is known in the art, frequency conversion circuits provide an output signal in response to at least one input signal having desired frequency components which are related to the frequency of the input signal. Generally, if the desired frequency components of the output signal are lower than that of the frequency of the input signal, the circuit is called a mixer or down converter. If the desired frequency components are higher than the frequency of the input signal, the circuit is called an up converter or multiplier. In either case, at least an input is fed to a non-linear device and the non-linear device either converts a portion of the input signal into a harmonic of the original input signal or the input signal is mixed with a second input signal to provide the output signal having the desired frequency components.
One problem with such circuits is that the non-linear element also generates in response to the input signal undesired frequency components. Typically, these frequency components are eliminated by filtering or phasing techniques. Filtering techniques although relatively straightforward are inadequate in applications when the bandwidth of the desired input frequency overlaps the bandwidth of the undesired input frequencies. In such cases, phasing techniques are generally used. One problem with phasing techniques is that generally couplers or baluns are used to provide a phase relationship between the signals. Couplers or baluns are in general narrowband devices, in particular, when fabricated as an integrated circuit. Moreover, such couplers or baluns are generally difficult to fabricate as integrated circuits. Accordingly, frequency conversion circuits are generally difficult to fabricate as integrated circuits, particularly, when phasing techniques are desired to be employed.